FIGS. 3 and 4 are explanatory views showing an axial flow fan having conventional five blades. FIG. 3 is a front view showing a conventional five-bladed axial flow fan, and FIG. 4 is a perspective view showing the conventional five-bladed axial flow fan. FIG. 14 is a view showing the spread of the wind generated when the conventional five-bladed axial flow fan is rotated. FIG. 16 is an explanatory view showing an axial flow fan having blades, the number of which is made larger than that of the blades of the conventional axial flow fan, while the shape of the blades remains the same.
Conventionally, an axial flow fan having three to five blades, in particular, an axial flow fan having five blades shown in FIGS. 3 and 4, is frequently used for general fans or the like. Since such an axial flow fan is easy to mold when it is produced, the shape of the fan has been unchanged for many years.
Furthermore, as shown in FIG. 14, in the case that the conventional axial flow fan having five blades, 30 cm in diameter, was rotated at 800 rpm, the diameter of the wind 19 generated from the axial flow fan was 50 cm at a position 3 m away from the front of the axial flow fan; the spread of the wind was almost negligible.
However, for example, the wind generated by a fan is frequently required to be distributed in a wide range, as in the case that such a fan is generally equipped with an oscillating function. Moreover, a blower is also used for a heater to distribute heat widely to a living space. Even in this case, heat transfer to the space is attained more easily when the air blowing range of the blower is wider.
In these circumstances, in the case of an appliance that uses an axial flow fan to blow air, it is frequently found that the fan is used more conveniently when the area of the wind generated during use is wider. For example, in the case that a huge axial flow fan is rotated, wind having a large area can be obtained. However, it is not realistic to install such a huge axial flow fan in the air blowing section of an existing appliance having an air blowing function because of the limited space in the appliance. Hence, it is preferably desired that the spread of the generated wind, i.e., the area of the wind, is increased without changing the diameter of the axial flow fan.
Furthermore, the volume of the wind generated from the axial flow fan becomes larger as the area of the blades thereof is larger in the case that the rotation speed is the same.
This means that, in the case that an axial flow fan, the area of the blades of which is larger than that of the conventional axial flow fan having five blades, for example, is rotated, the rotation speed of the axial flow fan can be made lower than that of the conventional axial flow fan having five blades, for example, to obtain the same volume of wind. This may lead to improvement in noise and power consumption.
However, at present, most of axial flow fans for use in fans, ventilators, heaters, etc. do not have more than five blades, and axial flow fans, the areas of the blades of which are significantly large, are not available.
This is mainly attributed to the fact that knowledge about fluid dynamics, etc. are required to design an axial flow fan having excellent efficiency and the fluid dynamics itself has many unknown aspects, whereby difficulties in design are anticipated easily and problems that can arise during high-volume production are anticipated.
For example, in the case that for the purpose of increasing the area of the blades of an axial flow fan, a shape 21 is formed by increasing the number of the blades of a generally-used five-bladed axial flow fan 10 while the shape of the blades remains unchanged as shown in FIG. 16, an overlap 22 is generated between the adjacent blades at the root sections of the blades as viewed from the front of the axial flow fan. This means that an undercut portion is generated when a two-part injection molding die for high-volume production is used for plastic molding, for example. This is unrealistic when it is assumed that high-volume production is carried out.
Moreover, as a solution to this problem, an idea of making the root sections of the blades slender so that the adjacent blades do not overlap can be conceived easily. However, a larger load is applied to a portion closer to the outer circumference of each blade during rotation, and the load is supported only by the root section that is made slender to prevent overlap, whereby there occurs a problem in the strength of the root section. Hence, it is preferably desired that the number and the area of the blades of the axial flow fan are increased while the strength of the axial flow fan itself is maintained high by adopting a shape that can be produced by two-part injection molding serving as a general high-volume production method.